Selective metal oxide removal

ABSTRACT

A metal oxide, utilized as a gate dielectric, is removed using a combination of gaseous HCl (HCl), heat, and an absence of rf. The metal oxide, which is preferably hafnium oxide, is effectively removed in the areas not under the gate electrode. The use of HCl results in the interfacial oxide that underlies the metal oxide not being removed. The interfacial is removed to eliminate the metal and is replaced by another interfacial oxide layer. The subsequent implant steps are thus through just an interfacial oxide and not through a metal oxide. Thus, the problems associated with implanting through a metal oxide are avoided.

RELATED APPLICATIONS

[0001] This application is related to the following applications thatare assigned to the assignee hereof:

[0002] U.S. patent application Ser. No. 09/851,206, by Hobbs et al.,entitled “Method for Making a Semiconductor Device;” and

[0003] U.S. patent application Ser. No. 09/574,732, by Hobbs et al.,entitled “Selective Removal of a Metal Oxide Dielectric.”

BACKGROUND OF THE INVENTION

[0004] In developing high-k dielectrics for use as gate insulatinglayers, the most common type of such high-k dielectrics have been metaloxides. These metal oxides have a significant higher dielectric constantthan the historic gate insulator of silicon oxide. In developing thesemetal oxides it has become a problem of doping the source/drain withimplants through such metal oxides. Thus, implanting through these metaloxides has been difficult because the metal oxides absorb and impede theprogress of the dopants that are being implanted. This can result inshallower source/drain regions, which is undesirable, and also in the PNjunctions being less abrupt. The energy of the implant can be increasedto achieve the desired depth of the source/drains but the abruptness ofthe PN junctions that are formed is still reduced. The disadvantage ofPN junctions that are not abrupt is increased resistance of the dopedregion due to the larger areas of low concentration of doping and alsohigher current leakage. The higher current leakage may result from thedepletion region extending further and enclosing more areas that havedefects. Further, the metal oxide must ultimately be removed in order tomake contact to the source/drains.

[0005] To overcome this disadvantage of implanting through a metaloxide, there have been attempts to remove the metal oxide prior toperforming the source/drain implants. Removal of this metal oxide,however, has been very difficult to control. If the etch of the metaloxide continues for too long, the underlying interfacial oxide isremoved and the underlying silicon is pitted. Thus there is a need toprovide a technique for removing metal oxides that does not result inpitting this silicon substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The present invention is illustrated by way of example and not bylimitation in the accompanying figures, in which like referencesindicate similar elements, and in which:

[0007] Shown in FIGS. 1-7 are sequential cross-sections of a portion ofa semiconductor wafer according to a preferred embodiment of theinvention; and

[0008] Shown in FIG. 8 is an apparatus useful in performing a portion ofthe method used in achieving the cross-sections shown in FIGS. 1-7.

[0009] Skilled artisans appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to helpimprove the understanding of the embodiments of the present invention.

DESCRIPTION OF THE INVENTION

[0010] In an embodiment of the invention a selective removal of a metaloxide is achieved by flowing anhydrous HCl (HCl) over a metal oxidelayer while it is receiving heat by radiation. The metal oxide iseffectively removed while the interfacial oxide layer underlying a gateelectrode is retained and protects the underlying silicon. The HCl flowsacross the wafer in the absence of being energized by high frequencyelectromagnetic waves. In this case, high frequency means radiofrequency or microwave radiation and is conveniently referenced as “rf”herein.

[0011] Shown in FIG. 1 is a device structure 10 comprising a siliconsubstrate 12, an interfacial oxide layer 14, a metal oxide layer 16, agate electrode 18, and an antireflective coating (ARC) 20. The areaimmediately under gate electrode 18 is the critical area for having ahigh degree of coupling and is the area where it is important for theinterfacial oxide layer 14 to be thin. Metal oxide layer 16 ispreferably hafnium oxide and is about 30 angstroms in thickness. Gateelectrode 18 may be any appropriate gate material and is preferablypolysilicon. ARC 20 is any appropriate antireflective coating materialand is preferably silicon-rich nitride. The interfacial oxide layer 14is much thinner under gate electrode 18 than in source/drain areas notunder gate electrode 18. Gate electrode 18 acts as a mask to oxygen andthus prevents additional growth of interfacial oxide layer 14 in thearea under the gate electrode.

[0012] Shown in FIG. 2 is device structure 10 after removal of metaloxide 16 in areas other than those under gate electrode 18. The portionof metal oxide 16 which remains is shown as metal oxide 22 which ispresent only under gate electrode 18. Metal oxide 16 is selectivelyremoved by placing it in a reaction chamber such as that shown in FIG.8.

[0013] Shown in FIG. 8 is an apparatus 26 comprising a reaction chamber24, a source of hydrogen chloride 28, a support 30, a radiation source32, and a wafer 34 that has device structure 10 present therein. Wafer34 is placed on support 30 and receives heat from radiation source 32.Coincident with wafer 34 receiving this radiation, HCl is flowed overwafer 34. There is no rf energy applied to the HCl. The result is theremoval of the metal oxide 16 that is exposed and the retention ofinterfacial oxide layer 14. It has been found that an effective range ofheat for wafer 34 is 600-800 degrees C. Even higher temperatures may beuseful, especially with very low partial pressures of HCl. As thetemperature increases to the high end of that range, there tends to bemore removal of the metal oxide under gate electrode 18. This would bethe type of undercutting that is commonly associated with isotropicetching. At the lower end of this range the removal of the metal oxideis slow. A good temperature that has been found to be effective inproviding a good rate of removal of the metal oxide and minimalundercutting of the metal oxide under gate electrode 18 is a temperatureof 650 degrees C. A good range of operation has been found to be 625-675degrees C. The combination of the radiation and the HCl is applied forabout 60 seconds at a pressure of 50 torr and at a flow rate onestandard liter per minute (SLM). Also flowing with the HCl is 9 SLM ofnitrogen (N₂), which operates as an inert gas. Other inert gases mayalso be effective.

[0014] Other combinations of time, temperature, pressure and flow ratemay also be found to be effective for effective removal of the metaloxide without significant undercutting. In practice, the HCl is flowedprior to the application of the heat from the radiation source 32. As analternative, heat could be achieved by convection or contact between ahot plate, for example, instead of by radiation as shown in FIG. 8. Anadvantage of radiation heating is that it provides for relatively fasterramp up times and ramp down times.

[0015] Shown in FIG. 3 is device structure 10 after removal of ARC layer20. ARC 20 may be removed by a dry etch or a wet etch. A dry etch mayresult in some bombardment of the sacrificial oxide layer 14, which isexposed, and potentially even reaching the silicon of substrate 12. Thispossibility may be avoided by using a wet etch. Another possibility isto remove ARC 20 prior to the selective removal of metal oxide 16. Hotphosphoric acid is an effective wet etch for an ARC layer made ofsilicon rich silicon nitride.

[0016] Shown in FIG. 4 is device structure 10 after a short, wet dip ofdevice structure 10 into hydrofluoric acid (HF). The purpose of the HFdip is to clean up any accumulated metal present in interfacial oxide 14by removing interfacial oxide 14 to leave thin interfacial oxide layer25 under gate electrode 18. The time is about 30 seconds. The time canbe adjusted according to the concentration of the HF. It should be longenough to insure the removal of the interfacial oxide layer 14 but notso long as to roughen the underlying silicon of substrate 12. Theinterfacial oxide layer 14 in the area that is not under gate electrode18 is in the range of about 15-35 angstroms. After the HF dip, thedevice structure 10 is rinsed with de-ionized water and may be exposedto air. The result is shown in FIG. 5 with device structure 10 thenhaving an interfacial oxide layer 27 in the area not under gateelectrode 18 having a thickness in the range of about 8-15 Angstroms.

[0017] Shown in FIG. 6 is device structure 10 after two implants. One ofthe implants is known as a halo implant and forms regions 40 and 42. Theother implants are the source drain extension implants and form sourcedrain regions 44 and 46. Regions 44 and 46 are of the oppositeconductivity type of that of substrate 12 whereas halo regions 40 and 42are of the same conductivity type as substrate region 12. Substrate 12is representative of either P-wells or N-wells and may be over aninsulating layer as is common for substrates known as SOI. Halo regions40 and 42 are to improve punchthrough.

[0018] Shown in FIG. 7 is device structure 10 after formation ofsidewall spacers 48 and 50 and the heavy source/drain implant. The heavysource/drain implant results in heavily doped contact regions 52 and 54.Portions of regions 52 and 54 immediately adjacent to the areaimmediately under gate electrode 18 are comparatively lightly doped. Thedoping of regions 54 and 52 is a significantly higher concentration thanthe doping of regions 40 and 42. Device structure 10 as shown in FIG. 7is thus a completed transistor.

[0019] Anhydrous halides other than HCl may provide similar results.Gaseous HF for example may remove the metal oxide effectively undersimilar conditions. A disadvantage of HF with respect to HCl is that HFis much less selective to silicon oxide. Thus, the interfacial oxide 14,after the metal oxide was penetrated, would be removed at a greater ratewith HF than it would be with HCl. Also metal fluorides are lessvolatile than metal chlorides which may result in less effective removalof the metal oxide. Thus, HCl is considered to be more desirable for usethan gaseous HF. Other exemplary gaseous halides that may be effectiveare HI, HBr, I₂, Br₂, Cl₂, and F₂. Hafnium oxide has a particularadvantage as a high-k dielectric as being stable with respect topolysilicon. It is less reactive with the polysilicon at the time thepolysilicon is deposited compared to many other high-k dielectrics.

[0020] The absence of rf activation of the HCl has been found to beadvantageous. A typical plasma etch, which uses rf, that includedchlorine gases was found to pit the substrate. The pitting is believedto be a result of bombardment, physical etching as distinguished fromchemical etching, of the surface by the rf-energized chlorine. Theabsence of the rf provides for just chemical removal of the metal oxide.This chemical removal of the present invention is believed to work bycausing a reaction between the gaseous halide, preferably HCl, and themetal oxide to generate a byproduct of a portion of the HCl and themetal in the metal oxide. Also the byproduct is volatile so that it iseasily removed from the reaction chamber and away from the wafer. Thissuccessful avoidance of the pitting is very beneficial because pittingcan result in non-uniform implant doping, which results in increasedresistance and/or other degradation in transistor performance.

[0021] Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus.

1. A method for forming a semiconductor device comprising: providing asemiconductor substrate; forming a metal oxide layer over thesemiconductor substrate; forming a patterned gate electrode over a firstportion of the metal oxide layer; and removing a second portion of themetal oxide layer by heating the semiconductor substrate and flowing ahalide-containing chemistry over the substrate while heating, whereinthe second portion of the metal oxide layer is adjacent to the firstportion of the metal oxide layer.
 2. The method of claim 1, wherein thehalide-containing chemistry further comprises hydrogen.
 3. The method ofclaim 2, wherein the halide-containing chemistry is HCl.
 4. The methodof claim 1, wherein the metal oxide layer is hafnium oxide.
 5. Themethod of claim 1, further comprising: forming an patterned ARC layerover the patterned gate electrode prior to the flowing of thehalide-containing chemistry; and removing the patterned ARC layer afterthe flowing of the halide-containing chemistry.
 6. The method of claim5, further comprising: forming a first interfacial oxide layer under themetal oxide layer; removing at least a portion of the first interfacialoxide after removing the second portion of the metal oxide layer;
 7. Themethod of claim 6, wherein removing at least a portion of the firstinterfacial oxide layer is performed using a chemistry containinghydrogen and fluorine.
 8. The method of claim 7, further comprisingforming a second interfacial oxide over the semiconductor substrate. 9.The method of claim 1, wherein the step of removing is furthercharacterized as being at a temperature of between about 625 degreesCelsius to 675 degrees Celsius.
 10. The method of claim 9, wherein thestep of removing is further characterized as being at a pressure ofabout 50 torr for approximately 60 seconds and a flow rate of thehalide-containing chemistry at about one SLM.
 11. The method of claim 1,wherein removing a second portion of the metal oxide layer is performedin a reaction chamber in the absence of rf activation.
 12. The method ofclaim 1, wherein heating is performed using a radiation source.
 13. Amethod of removing a metal oxide layer that is over a semiconductorsubstrate, comprising: placing the semiconductor substrate into areaction chamber; heating the metal oxide layer; flowing achlorine-containing chemistry while heating, wherein thechlorine-containing chemistry reacts with a portion of the metal oxidelayer to create a byproduct, wherein the byproduct comprises an elementfrom the metal oxide layer; and removing the byproduct from the reactionchamber.
 14. A method for forming a semiconductor device comprising:providing a semiconductor substrate; forming a metal oxide layer overthe semiconductor substrate comprising hafnium and oxygen; removing aportion of the metal oxide layer by heating the semiconductor substrateusing radiation and flowing a chemistry containing hydrogen andchlorine.
 15. The method of claim 14, wherein heating the semiconductorsubstrate is at a temperature between about 625 degrees Celsius to 675degrees Celsius.
 16. The method of claim 14, wherein the semiconductorsubstrate comprises silicon.
 17. The method of claim 16, furthercomprising: forming a first interfacial oxide layer under the metaloxide layer; removing at least a portion of the first interfacial oxideafter removing the portion of the metal oxide layer;
 18. The method ofclaim 17, wherein removing at least a portion of the first interfacialoxide layer is performed using a chemistry containing hydrogen andfluorine.
 19. The method of claim 18, further comprising forming asecond interfacial oxide over the semiconductor substrate.
 20. Themethod of claim 14, wherein removing a second portion of the metal oxidelayer is performed in a reaction chamber in the absence of RFactivation.
 21. A method of forming a metal oxide comprising: providinga semiconductor substrate; forming a metal oxide layer over thesemiconductor substrate; removing a portion of the metal oxide layer byheating the semiconductor substrate and flowing a gaseous halide. 22.The method of claim 21, wherein the gaseous halide comprises hydrogen.23. The method of claim 22, wherein the gaseous halide is HCl.
 24. Themethod of claim 22, wherein the gaseous halide is HF.
 25. The method ofclaim 21, wherein the metal oxide contains hafnium and oxygen.
 26. Themethod of claim 21, wherein heating the semiconductor substrate is at atemperature between about 625 degrees Celsius to 675 degrees Celsius.27. A method of selectively removing a metal oxide layer from asemiconductor substrate, wherein the metal oxide layer has an exposedportion and a portion under a gate electrode comprising the step offlowing gaseous HCl, in the absence of rf activation, over the substratewith the substrate heated to between 600 and 800 degrees Celsius. 28.The method of claim 27, wherein the substrate is heated to between 625and 675 degrees Celsius.
 29. The method of claim 28, wherein the metaloxide is hafnium oxide.
 30. The method of claim 29, wherein thesubstrate is heated by radiation.
 31. The method of claim 30, whereinthe metal oxide overlies an oxide layer.